1. Field of the Invention
The present invention relates to a hybrid-powered vehicle equipped with an internal combustion engine and a motor-generator. The present invention relates to a technology for realizing a hybrid-powered vehicle by mounting, as a power source, an internal combustion engine and a motor-generator on the vehicle having a power transmission system including a torque converter and a multistage transmission. The present invention relates to an improvement of drivability of a hybrid-powered vehicle and a reduction of energy loss caused by a torque converter of the vehicle.
2. Description of Related Art
In order to effectively utilize torque of an internal combustion engine of a vehicle as a power source thereof in a wide speed range, a transmission is provided between the power source and an axle of the vehicle. Besides, the vehicle having an automatic transmission, which does not require gear shift by a clutch pedal during a driving, has become popular recently and, nowadays, almost all drivers are familiar with such automatic transmission.
Although a planet gear type transmission or a continuously variable type transmission is known as a transmission for such automatic transmission, an automatic transmission system having a hydraulic torque converter and a multistage transmission combined in series with the hydraulic torque converter is popularly used. The multistage transmission has a structure in which a mechanical speed change operation is automatically controlled according to a control signal from a program control circuit. The transmission of this type has been improved on its design and manufacture and a structure of a change lever in a driver's seat, etc., can be designed very reasonably with high operability. Under the circumstances, it becomes possible to provide, at a reasonable price, a highly reliable transmission, which substantially improves drivability of a vehicle.
JP 2000-203287A (Toyota) discloses a hybrid-powered vehicle employing a transmission including a hydraulic torque converter and a mechanical multistage transmission combined with the hydraulic torque converter. A main object of the transmission disclosed in JP 2000-203287A is to prevent overrunning of a rotary shaft of a motor-generator during regenerative braking. So far, any hybrid-powered vehicle mounting a transmission including a hydraulic torque converter combined with a mechanical multistage transmission is not sold as a utility vehicle.
In a hydraulic torque converter, a slip occurs between an input shaft and an output shaft of the torque converter when a vehicle is accelerated or decelerated or when a speed change operation is performed. Moreover, such slip occurs when power is transmitted between the input shaft and the output shaft of the torque converter. Due to the slip, it is possible to smoothly perform a torque conversion correspondingly to a change of speed. However, the torque converter is heated by this slip, so that power energy is lost. In order to minimize the slip of the hydraulic torque converter connected in series with the mechanical multistage transmission and the power transmission system, a mechanism for mechanically locking up the input shaft and the output shaft of the torque converter when a difference in rotation speed between the input shaft and the output shaft thereof becomes smaller than a predetermined value has been considered. With using such mechanism, steady heat generation of the torque converter is prevented. Therefore, this mechanism has been utilized widely.
The hydraulic torque converter mentioned above is means for controlling the input shaft and the output shaft by utilizing the slip therebetween in such a way that the difference in rotation between the input shaft and the output shaft becomes smaller. Therefore, so long as a vehicle utilizes this torque converter, increase of specific fuel consumption of the vehicle is theoretically indispensable. That is, the utilization of the hydraulic torque converter and the substantial improvement of fuel economy by employing the hybrid powers of the internal combustion engine and the motor generator contradict each other. Therefore, it can be said so far that a transmission including a hydraulic torque converter is not employed in a hybrid-powered vehicle.